1. Field of the Invention
The present invention relates to an ultrasonic delay line in which electric signals are translated into sound waves and the translated sound waves are transmitted in a delay medium so that the electric signals are delayed.
2. Discussion of Background
The ultrasonic delay line constructed as above-mentioned has widely been used as an element for the circuit of a TV receiver, a video tape recorder (VTR), a video camera and so on.
In such ultrasonic delay line, there are main sound waves transmitted through a desired path in the delay medium and other sound waves transmitted through paths other than the desired path. Accordingly, when the other sound waves are picked up as spurious signals by an output transducer, an image reproduced, for example by the VTR, may be adversely affected.
To avoid this problem, there has been proposed a method of attenuating the spurious sound waves by attaching at least one absorbing material onto the main surface of the delay medium in parallel to the transmitting direction of the sound waves and side surfaces perpendicular to the main surface, or by roughening the main surface and the side surfaces.
Also, there has been proposed a method of attenuating 2.tau. and 3.tau. spurious signals by providing at least one absorbing material in the desired path, as shown in commonly owned Japanese Examined Patent Publication No. 19942/1977. The 2.tau. spurious signal is a sound wave that is returned to the input transducer through the desired path after it has been once picked up by the output transducer; hence the delay time of the sound wave is twice the desired delay time .tau.. The 3.tau. spurious signal is such a sound wave that the 2.tau. spurious signal is again transmitted through the desired path to be picked up by the output transducer.
In the above-mentioned conventional spurious sound wave attenuating methods, an attempt has been made only to remove the fundamental frequency component of a sound wave (hereinbelow, referred to as the main signal) transmitted in the delay medium. Namely, measures have been taken to remove the fundamental frequency component of the spurious signals transmitted through paths other than the desired path and the fundamental frequency component of the 2.tau. and 3.tau. spurious signals transmitted through the desired path.
However, sound wave signals transmitted in the delay medium generally have harmonic components. For instance, a frequency characteristics diagram of FIG. 2 shows superposition of a spurious signal having the third harmonic component which has a frequency f.sub.3 (exemplified as frequencies from 13 MHz to 16 MHz in FIG. 2) three or four times higher than the center frequency f.sub.0 (exemplified as 4.3 MHz in FIG. 2) of a main signal pass band. When the spurious signal having such harmonic component is of a fairly high level (for example, a level 20 dB lower than the main signal level in FIG. 2) and is superposed on the main signal, and if a video camera is used under this condition, stripe-like noise appear in a reproduced image thereby adversely affecting image reproduction.